507 



attached anteriorly. It was supposed that the stimulus thus received 

 by the sensory epithelial cells might be transmitted to appropriate 

 nerve-cells in the brain, and that the deviations of the long axis of 

 the body from the normal position might thus be regulated by reflex 

 action; and a comparison was made with the function of the semi- 

 circular canals of vertebrates, and also with that of the "Statocysts" 

 of many invertebrates, which serve by means of mechanical stimuli, 

 due in this case to the action of gravity, automatically to regulate 

 the orientation of the body. 



The fact that man, almost the only vertebrate which has assumed 

 the erect posture, and one of the few which have completely lost the 

 tail, also has the sub-commissural organ in a more reduced condition 

 than in any other case known, while the semi-erect and almost tail- 

 less chimpanzee is in this respect in an intermediate condition between 

 man and the lower vertebrates, seems clearly to indicate that the 

 function of the sub-commissural organ is in some way connected with 

 the position of the body and the flexibility of the vertebral column 

 (including of course the tail). 



Considering that the sub-commissural organ, with which Reissner's 

 fibre, when present, is invariably connected, is reduced to a mere 

 vestige in the adult man, it seems extremely improbable that man 

 possesses any Reissner's fibre at all. The only primates in which the 

 presence of Reissner's fibre has yet been demonstrated are Macacus 

 cynomolgus and M. rhesus (Horsley, 1908), and it is perhaps signi- 

 ficant that these are both tailed and non-erect forms. 



We may perhaps point out that the suggesstion thus put forward 

 as to the possible function of Reissner's fibre and the sub-commissural 

 organ is not inconsistent with the view previously maintained by one 

 of us (Dendy, 1902), to the effect that in the Ammocoete the latter 

 may serve by ciliary action to promote the circulation of the cerebro- 

 spinal fluid. 



We should like to express our indebtedness to our colleagues 

 Prof. Waterston, Prof. Peter Thompson, and Mr. Frazer, for placing 

 at our disposal some of the material upon which this paper is based, 

 and Mr. R. W. H. Row for making the photomicrographs used in 

 illustration. 



Literature referred to. 



1902. Dendy, On a Pair of Ciliated Grooves in the Brain of the 

 Ammocoete apparently serving to promote the Circulation of the 

 Fluid in the Brain-cavity. R. Soc. Proc, Vol. 69. 



